Theory and research on the gut-brain-axis emphasize complex interactions between the gut microbiota, immunological and hormonal responses, brain function, brain structure, as well as resulting behavioral manifestations, such as cognitive functions and mental illness. Probiotics are living micro-organisms that change the composition of the gut microbiota and hypothetically have a positive effect on the host's general health and well-being. Probiotic bacteria naturally occur in foods such as Sauerkraut, olives, and dark chocolate, and are currently also added to industrial products such as yogurt.

Regarding the effect of probiotics on brain structure and function, animal studies have shown that the administration of probiotics in mice and rats was linked to neurogenesis in the hippocampus and an improvement of associated cognitive functions. The majority of these studies applied probiotics for 4 weeks. The substances used in these studies were often composed of several bacterial strains, suggesting that the neurogenic effect may not be reducible to a specific type of probiotic bacteria.

Probiotics seem to be effective in improving memory abilities, including spatial and non-spatial memory, both in rodents and humans. Moreover, specifically regarding the beneficial effect of probiotics on anxiety, depression and stress, preliminary evidence in humans is compelling. However methodologically sound (randomized-controlled trial [RCT], 'blind') studies are still lacking.

To sum up, the present study is going to be the first RCT with human participants that investigates structural and functional changes of the hippocampus through probiotic bacteria, using Magnet Resonance Imaging (MRI). In addition, the study aims at advancing research in the field by investigating the effects of probiotics on a broad spectrum of cognitive functions, particularly those associated with hippocampal activity (e.g. spatial memory, verbal memory). Furthermore, effects on several types of mental illness (e.g. anxiety, depression, stress) will be studied. Potential translatory mechanisms that may promote the aforementioned effects will be examined, i.e. changes in immunological parameters, 'brain derived neurotrophic factor' (BDNF), and oxytocin.

A double-blind RCT intervention study will be conducted to investigate the effect of a probiotic dietary supplement vs. a placebo, taken by healthy human participants (N = 60) for 28 consecutive days. Both substances are administered in doses of 4.4g (powder) per day.

The probiotic dietary supplement used in the present study is a freely available product (in powder form) sold under the name of Vivomixx®. Per 4.4g the powder contains 450 billion bacteria, composed of eight bacterial strains: Lactobacilli (L. paracasei, L. plantarum, L. acidophilus and L.delbrueckii subsp. bulgaricus), Bifidobacteria (B. longum, B. infantis, B. breve), and Streptococcus thermophiles.

This product was chosen, because a similar supplement with an identical composition of bacteria as Vivomixx® was found to be linked to hippocampal neurogenesis in mice. The placebo powder has a similar taste and consistency as the verum (Vivomixx®) powder, but contains no probiotic bacteria.

First, participants will be screened online for eligibility (T0). Before and after the 28-day intake period, participants are asked to perform several cognitive tests and fill out questionnaires (T1, T2) at our laboratory (ca. 2 - 2.5 hours). Afterwards, blood samples will be drawn (ca. 5 minutes) and a magnet resonance tomography (MRT) session (ca. 1 - 1.5 hours) will be conducted, where participants will perform an object pattern separation task in the scanner (T1, T2). Furthermore, a follow-up after 2 months (T3), including the same assessments as at T1 and T2, will be conducted.

During the intake period participants will fill out a brief questionnaire on a weekly basis, which includes an assessment of state depression and anxiety, defecation (control item to rule out adverse effects), recent (start of) intake of medication, recent (onset of) intake of antibiotics, and recent inter-current illness (e.g. influenza). In addition, participants will record their food intake (protocol) for one week (7 consecutive days) during the intake period.

Inclusion Criteria:

• healthy participants
• age 18-40
• informed consent for all parts of the study (including MRT)
• none of the exclusion criteria (see below) fulfilled

Exclusion Criteria:

• age < 18 or > 40 years
• pregnancy or breastfeeding
• left-handedness
• degenerative or inflammatory diseases of the central nervous system
• severe cognitive/ neuropsychological impairment
• severe pain syndrome or other severe organic diseases
• epilepsy
• (past or present) psychiatric disorders
• neurological disorder
• severe diabetic polyneuropathy
• malignancies/ cancer
• cardiac insufficiency
• arterial hypertension
• heart attack/ stroke
• severe hepatic or renal insufficiency
• diseases of the hemopoietic system
• alcoholism/ drug addiction
• medical history of severe allergic or toxic reactions
• current participation in drug trial
• doubts about legal capacity/ capability of understanding
• referral to institutions based on court/ official order
• treatment with centrally acting medication (e.g. antipsychotics, antiepileptics, antidepressants, etc.)
• non-removable metal pieces (aneurysm clips, artificial limbs, etc.) or implanted electronic devices (pacemaker, osmotic or other implanted pumps, cochlear implants, etc.)
• claustrophobia
• acute (respiratory) infection, physical uneasiness
• tattoos in the head region, permanent make-up
• non-removable piercings
• chronic/ acute gastrointestinal diseases
• vegetarianism, veganism
• current intake of antibiotics or intake of antibiotics at any time point during the last 2 months
• lactose intolerance
• general focus on probiotic diet

double blind